The invention relates to a process for the cooling of waste gas bends and to an apparatus for the performance of the process, consisting of a waste gas bend with a cooled pipe wall.
Contrary to waste gas flues, waste gas bends for industrial furnaces are exposed to greater thermal loads than any other components of the waste gas system in industrial plant. A waste gas bend serving to deflect the flow of waste gas by an angle of about 90.degree. in direct contact with the cover of the furnace is exposed to incomparably high thermal and mechanical stresses which can in no way be compared to the problems arising with waste gas flues, which mainly take a vertical course and only have slight bends, in addition to which the waste gas temperatures occurring therein are far lower. The waste gas bends in industrial furnaces thus still have the shortest length of life, as the walls are exposed to waste gas temperatures of 1400.degree.-1600.degree., leading to very high differential stresses and to local disintegration of material and causing the waste gas bend to undergo premature destruction.
Waste gas bends with double walls serving to convey a cooling medium have hitherto been made from sheet metal segments with angular portions. These known waste gas bends have certain disadvantages which impede the flow required for the deflection of the hot waste gases, since considerable turbulence occurs at each angular joint, causing localisation of heat, phenomena which cannot be controlled by means of the known cooling systems. Bubbles of steam cannot be prevented from forming at these points, inevitably resulting in the destruction of the waste gas bends.
The further causes of this premature destruction of the known types of waste gas bend reside in the fact that the hottest zones, which in general are situated in those portions of the waste gas bend in which the cross section is the maximum, are transmitted by cooling media to the wall parts of the cooler sectors of the cross section. With waste gas flues with the known type of pipe wall, for instance, it has already been suggested that the coolant should be allowed to flow through the pipe wall subdivided into vertical sections, the speed of the coolant in the individual sections being regulated in accordance with the quantity of heat occuring such as disclosed in the German patent No. 1408919.
The teaching provided by the said patent, however, is not applicable to waste gas bends, as the flow of coolant under the force of gravity is not capable of controlling local peak temperatures such as occur in waste gas bends and of removing the heat which accordingly occurs. One of the main drawbacks, however, is the fact that the intensified removal of the heat in certain portions of the waste gas cross section is transmitted over the entire length to other parts of the wall of the waste gas flue. This causes, in the sections of the waste gas flue as well, stresses due to temperature differences, where otherwise advantageous distribution of heat prevailed in the pipe wall. The temperature differences in this known apparatus were not balanced out over the complete waste gas system but merely transferred to other parts of the wall, so that the destructive effect due to heat differences could not be eliminated. Furthermore, the flow cross section of the cooling circuit in this suggested system is constricted by control valves, so that the elimination of heat, in the temperature range in which the bubbles of steam form, cannot be ensured.
The purpose of the present invention is to develop a process which solves the problem connected with the required cooling in waste gas bends, enabling local excess heating in the wall of a pipe bend to be avoided and reliably preventing the production of stresses due to temperature differences in the entire pipe wall surface, and also to design an apparatus enabling the process covered by this invention to be carried out.